Method of producing carbon nanoparticles

ABSTRACT

A method of producing carbon nanoparticles comprises the steps of: passing a gaseous carbon source through a heated reactor; and adding catalyst supported on substrate particles or thermally decomposable catalyst precursor supported on substrate particles to the heated reactor to form a fluidised bed; such that carbon nanoparticles are formed in the heated reactor.

FIELD OF THE INVENTION

The present invention relates to a method of producing carbonnanoparticles, and to carbon nanoparticles so produced.

BACKGROUND OF THE INVENTION

Carbon nanoparticles may be produced by various routes, includingcatalytic vapour deposition (CVD), arc discharge and laser ablation.

The CVD route has advantages of low cost and scalability. There hastherefore been significant interest in this route.

Typically, in the CVD route, a gaseous carbon source such as ahydrocarbon or carbon monoxide is decomposed by a metallic catalyst in aheated reactor under suitable reaction conditions. Carbon nanoparticles(for example carbon nanotubes) are deposited.

The catalyst may be either supported by a substrate or suspended in thegas stream. The catalyst may be introduced into the reactor in thefollowing ways:

-   -   1. Placing a supported catalyst or catalyst precursor (e.g.        ferrocene, iron pentacarbonyl) into the reactor and then        introducing a gaseous carbon source into the reactor. This is        the fixed bed method. The supported catalyst may be made by        sputtering catalyst metal onto a substrate, by oxidation of a        metal salt followed by reduction (WO 00/73205), by impregnation        of a metal salt into a high surface area substrate [Geng 02], or        by a sol-gel reaction using precursors containing the catalyst        elements and the support materials [Su 00, Flauhaut 99], or by        in situ thermal decomposition of a supported catalyst precursor.    -   2. Introducing the catalyst in the form of a precursor directly        into the gaseous carbon source in the heated reactor to produce        catalytic metal particles in situ by thermal decomposition. The        metal catalyst is suspended in the reaction gas mixture [WO        00/26138].    -   3. Introducing the catalyst in the form of a precursor directly        into the gaseous carbon source in the heated reactor to deposit        catalytic metal particles onto solid supports held within the        reactor [Singh 03].    -   4. Introducing the catalyst and substrate into the furnace,        where upon they react, as described in WO02/092506.

The most commonly used process for synthesizing nanoparticles is thefixed-bed method. In the fixed-bed method, the supported catalyst isheated slowly within the heated reactor. Some fixed-bed supportedcatalyst systems produce nanotubes, while others yield only amorphouscarbon or carbon capsulated metal particles. There is often failure toproduce carbon nanotubes, and in particular failure to producesingle-walled nanotubes [Li and summary of WO 0017102].

A promising process for large-scale synthesis of carbon nanotubes is thefluidised bed method. Fluidised-bed processes are well-established inchemical engineering. Such processes have the advantage of enhancinggas-solid mixing so as to increase reaction efficiency and provideuniform products.

Fluidised bed methods have been used for production of multi-walledcarbon nanotubes. These methods have been carried out by introducingsupported catalyst into a heated fluidised bed reactor followed by slowheating to a synthesis temperature [Wang, Carbon].

Some workers use an additional reduction step in hydrogen prior to thenanotube synthesis reaction [Wang,Bachilo]). Recently, Bachilo et al.and Mauron et al. have reported the production of single-wallednanotubes from salt impregnated silica which was oxidised and thenreacted [Bachilo, Mauron].

Fluidised bed methods also suffer from the disadvantage which applies tothe fixed-bed method of failure to produce carbon nanotubes and inparticular failure to produce single-walled carbon nanotubes.

SUMMARY OF THE INVENTION

In a first aspect, the present invention provides a method of producingcarbon nanoparticles, comprising the steps of:

-   -   passing a gaseous carbon source through a heated reactor; and    -   adding catalyst supported on substrate particles or thermally        decomposable catalyst precursor supported on substrate particles        to the heated reactor;    -   maintaining a fluidised bed of the substrate particles in the        heated reactor; and    -   forming carbon nanoparticles in the heated reactor.

The particles bearing the catalyst or precursor are added to the heatedreactor in the presence of the gaseous carbon source. Preferably, theparticles are thereby rapidly heated from a temperature at which theycan be stored without deterioration in their nanoparticles formingproperties to the temperature of the heated reactor. In an exampledescribed below is of the order of 10²-10³° C./min. More generally, aheating rate between 10 and 10⁴° C./min should be acceptable, but morepreferably it should be above 10²° C./min. Preferably, the particles aresubjected to said rapid heating from a starting temperature not above300° C., more preferably not above 100° C., e.g. from around roomtemperature. Suitably, the heating time from the safe startingtemperature to the temperature of the heated reactor is from 0.01-60seconds, more preferably not exceeding 20 seconds. Generally, thedifference in temperature between the storage of the particles beforeinjection and the heated reactor should be from 100 to 1200° C., morepreferably 500 to 1000° C.

Preferably, the catalyst or catalyst precursor supported on substrateparticles is introduced into the heated reactor via a gravity-feedhopper. Alternatively or additionally, the catalyst or catalystprecursor supported on substrate particles may be introduced into theheated reactor via an injection gas flow. Thus, the injection gas flowmay be used to entrain and carry particles released from a hopper tofall into the gas flow, or the gas flow may be used to lift particlesfrom a bed of particles to carry them into the heated reactor.

It may be that the injection gas flow reverses the direction of gas flowthrough the heated reactor or in a portion thereof during injection.

The injection gas is suitably an inert gas but may also be or maycomprise a gaseous carbon source.

The reactor heated reactor is suitably at a temperature between 500 and1200° C., more preferably at a temperature between 700 and 900° C.

When a catalyst precursor is present it is suitably a metal salt, anorganometallic species or a metal carbonyl. Such a catalyst precursormay comprise one or more of nickel, iron, molybdenum, platinum andcobalt. Suitably, the catalyst precursor is a metal salt and comprises acounter ion consisting of nitrate, stearate, formate, oxalate, acetateor chloride. The organic counter ions are preferred, for instance C₂ toC₃₀ carboxylate.

The carbon nanoparticles may contain a non-carbon dopant such asnitrogen.

The gaseous carbon source is suitably one or more of acetylene, alcohol,alkane, alkene, CO, benzene, toluene, xylene, cumene, ethylbenzene,naphthalene, phenanthrene, anthracene, formaldehyde, acetaldehyde, oracetone.

Preferably, the gaseous carbon source is mixed with a diluent gas andpreferably the mixture of these gases fluidises the bed of substrateparticles. The diluent gas is preferably one or more of hydrogen,ammonia, nitrogen, helium and argon.

The ratio of gaseous carbon source to diluent gas is preferably reducedwhile the catalyst or catalyst precursor supported on substrateparticles is introduced into the heated reactor, e.g. so that theproportion of the gaseous carbon source in the mixture drops by a from20 to 100%, so that for instance if during nanoparticles production theratio is say 1:2 carbon source to diluent, during particle addition theamount of carbon source fed might be reduced so that the ratio is from2/3:2 (33% reduction) down to 0:2 (100% reduction). More preferably saidreduction might be by from 40 to 60%.

The substrate particles may comprise or consist of one or more ofsilica, alumina, MCM (a family of mesoporous aluminosilicate molecularsieve materials, including MCM-41), and magnesium oxide. Suitablytherefore, the substrate particles comprise a halide, nitrate, sulphate,carbonate, aluminate, aluminium chloride, arsenate, arsenite, borate,chromate, fluoroaluminate, silicate, sulphide, telluride, tungstate,vanadate or phosphate of a Group 1 or Group 2 metal. The Group 1 orGroup 2 metal may be lithium, sodium, potassium, calcium or magnesium.

Suitably, the average dimension of the substrate particles is between 20microns and 1 mm, more preferably between 40 microns and 200 microns.

The method according to this first aspect of the invention preferablyfurther comprises the step of removing nanoparticles from the heatedreactor. This may be done by the use of vacuum to suck out the particlesbearing the nanotubes or by the use of pressure, e.g. the use of a gasjet to blow the particles off the top of the bed for collection.

The process is preferably operated continuously with continuous orrepeated introduction of catalyst or catalyst precursor supported onsubstrate particles and optionally simultaneous similarly continuous orcontinual removal of nanoparticles.

Alternatively, the method is operated non-continuously with alternatingbatch wise introduction of catalyst or catalyst precursor supported onsubstrate particles and removal of nanoparticles.

The carbon nanoparticles produced may be nanotubes and/or nanofibres.Subtle variations in conditions can be used to produced nanoparticlesselectively of a desired kind. The nanotubes may be single-wallednanotubes or multi-walled nanotubes.

In an alternative aspect, the invention includes a method of producingcarbon nanoparticles, comprising the steps of:

-   -   passing a non-carbon-containing gas through a heated reactor;        and    -   adding catalyst or catalyst precursor supported on substrate        particles to the heated reactor;    -   maintaining a fluidised bed of said substrate particles in the        heated reactor;    -   passing a gaseous carbon source through the heated reactor; and    -   forming carbon nanoparticles in the heated reactor.

In preferred methods according to either aspect of the invention,efficiency is enhanced by the fact that the supported product particleshave a lower density that the supported catalyst particle, and hence arepreferentially carried out of the reactor by the fluidising gas flow.

DETAILED DESCRIPTION OF THE INVENTION

The invention will be further described with reference to a preferredembodiment of the invention (Example 2) and to the figures, in which:

FIG. 1 shows in schematic sectional side elevation an apparatus for usein the invention.

FIG. 2 shows Raman spectra of the products synthesized in (a) Example 2and b) Comparative Example 1.

FIG. 3 shows SEM micrographs of the products synthesized in (a) Example2 and b) Comparative Example 1.

The apparatus shown in FIG. 1 comprises a resistance tube furnace 10extending vertically and annularly surrounding a vertically runningquartz tube 12 having upper and lower end caps 14, 16. Within the quartztube 12 is an inner quartz tube 18 running coaxially therewith from thelower end cap 16 on which it is supported and stopping short of theupper end cap 14. Approximately half way along its length, the innerquartz tube 18 has a disc 20 of porous silica frit bridging across itsbore. An inlet tube 22 for the introduction of a mixture of gaseouscarbon source and diluent gas extends through the lower end cap 16axially into the lower end of the inner quartz tube 18. An outlet tube24 for venting gas from the reactor extends from the annular spacebetween quartz tubes 12 and 18 through the lower end cap 16.

An inlet tube 26 extends axially through the upper end cap 14 to reachdown into the upper part of the inner quartz tube 18. A hopper 28 forthe gravity feed of substrate particles is connected via a ball valve 30to a port at the top of a horizontal run of the tube 26 and a side armof said tube leading to said port is connected to a supply 32 of carriergas.

In use, the furnace is heated to heat the quartz tubes 12 and 16 to thedesired nanotubes forming reaction temperature and a flow of carboncontaining gas and diluent gas mixture is established through inlet 22.Thereafter, substrate particles are dropped from the hopper 28 anddisplaced by a flow of carrier gas from the side arm of tube 26 to fallinto the reaction zone where they are supported on the frit 20 and forma fluidised bed 34. Carbon nanoparticles then form on the substrateparticles.

The invention will be further described with reference to the followingnon-limiting examples.

EXAMPLES Comparative Example 1

Nickel formate/silica gel particles were prepared by impregnating poroussilica gel particles (50 micron in diameter) with a nickel formateaqueous solution. A nickel loading of 3.0 wt % was obtained.

100 mg of the supported catalyst particles were placed onto the bed of afluidised bed reactor containing a porous frit at room temperature. Thereactor was purged with argon and was then heated at 10° C./min to thesynthesis temperature of 860 C.

The supported catalyst particles were then fluidised by passing a streamof methane and argon (ratio 1:2) through the bed at a flow rate of 2.0l/min. After 20 min and subsequent cooling of the system, the productswere collected from inside the fluidised reactor and were characterizedby Raman spectrometry and scanning electronic microscopy (FIG. 1 b),FIG. 2 b)). This showed that only amorphous carbon was formed on thesurface of the silica gel particles.

A similar synthesis was conducted in a horizontal reactor by a fixed-bedmethod. An identical supported catalyst was placed in an aluminacrucible then heated to the reaction temperature in the reaction gasmixture described above. Again, in this case, only amorphous carbon wasformed.

Example 2

A hot-injection synthesis was conducted using the same supportedcatalyst of Example 1.

The supported catalyst was held outside the reactor under an inert argonatmosphere whilst the fluidised bed reactor was heated to 860° C. Oncethe reactor had reached this temperature, the supported catalystparticles were blown into the top of the vertical reactor using argon(600 ml/min) as the carrier gas.

During addition of the supported catalyst, a methane-argon mixture(ratio 1:2, 2.0 l/min) was kept flowing through the bed. The catalystparticles were fluidized on the bed in a 1:1 methane-argon mixture, at aflow rate of 2.0 l/min, at 860° C. for 20 min.

As the catalyst was exposed to the carbon source at the hightemperature, an immediate colour change of the catalyst particles fromtheir original green colour to brown or black was observed on thoseparticles which were swept out of the fluidised bed reactor.

SEM observation (FIG. 1 a)) of the black products collected inside thefluidized bed reactor revealed a distribution of fibrous carbon productson the silica gel particles, and Raman analysis (FIG. 1 b)) showed thatthese particles were single walled nanotubes, as demonstrated by thepresence of a strong G band at 1585 cm-1 and radial breathing modes atthe low frequencies.

Example 3

The supported catalyst injection method of Example 2 was carried outusing pure methane rather than a mixture of methane and argon as theinjection gas. The synthesis was carried out under the same conditionsas Example 2, using 1:1 methane-argon. Multi-walled carbon nanotubeswere grown on the surface of the silica-gel particles rather thansingle-walled nanotubes.

The advantages of the method of Example 2 include:

-   -   1. The method improves the efficiency of the catalyst, that is,        the percentage of catalyst which produces single-walled        nanotubes.    -   2. The addition and subsequent removal of the catalyst while the        reactor is hot means that the plant is run more efficiently than        a conventional fluidised bed reactor plant.    -   3. The plant can be run in a continuous or semi-continuous mode.        A conventional fluidised bed reactor plant is run in a batchwise        mode.

Without wishing to be bound by theory, the applicants believe that goodresults are achieved in the method of Example 2 for the followingreasons.

In the fixed-bed method, catalyst particles are formed by thermaldecomposition of catalyst precursor during heating. The nature of thecatalyst particles is affected by the rate of heating. In particular,slow heating may result in larger catalyst particles because of slowdecomposition of the catalyst precursor and ripening of the catalystparticles on the substrate surface after decomposition. This can lead tofailure to produce carbon nanotubes, and in particular to failure toproduce single-walled nanotubes whose growth requires catalyst particlesof similar diameters to the nanotubes (a few nanometres) [Li and summaryof WO 0017102]).

In order to produce carbon nanotubes, it is necessary to form catalystparticles of small size. This can be achieved by rapid heating of thesupported catalyst in a highly dispersed state. This leads to theformation of small catalyst particles due to the impeded decompositionof the catalyst precursors. The impeded

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Rapid heating of a catalyst precursor has been used in a floatingcatalyst method to synthesize nanotubes. In this method, a preheated gaswas injected into a heated reactor with a catalyst precursor from acooled nozzle [WO 00/26318]. No catalyst support was used. In thepreferred embodiment of the present invention, the catalyst supportplays an essential role.

Whilst the invention has been described with reference to a preferredembodiment, it will be appreciated that various modifications arepossible within the scope of the invention.

REFERENCES

Luo—G. Luo, Z. Li, F. Wei, L. Xiang, Xiangyi Deng, Yong Jin. Catalystseffect on morphology of carbon nanotubes prepared by catalytic chemicalvapor depostion in a nano-agglomerate bed, Phyisca B, 323, 2002, 314-317

Wang—Y. Wang, Fei Wei, Guansheng Gu, hao Yu, Agglomerated carbonnanotubes and its mass production in a fluidised-bed reactor, Physica B,323, 2002, 327-329

-   -   The large-scale production of carbon nanotubes in a        nano-agglomerate fluidized-bed reactor, CPL, 364 (5-6), 2002, pp        568-572 Yao Wang, Fei Wei, Guohua Luo, Hao Yu and Guangsheng Gu

Carbon—Vengoni D, Serp P, Feurer, R, Yolande K, Vahlas C, Kalck P,Parametric study for the growth of carbon nanotubes by catalyticchemical vapor deposition in a fludised bed reactor, Carbon 40, 2002, pp1799-1807

“Narrow (n,m)-Distribution of Single-Walled Carbon Nanotubes Grown usinga Solid Supported Catalyst” S. M. Bachilo, L. Balzano, J. E. Herrera, F.Pompeo, D. E. Resasco and R. B. Weisman, J. Am. Chem. Soc. Submitted(available at http://www.ou.edu/engineering/nanotube/publications.html)

Li—Li Y, Kim W, Zhang Y, Rolandi M, Wang D, Dai H. Growth ofsingle-walled carbon nanotubes from discrete catalytic nanoparticles ofvarious sizes. Journal of Physical Chemistry B 2001;105:11424-11431

Geng 02—Geng J F, Singh C, Shephard D S, Shaffer M S P, Johnson B F G,Windle A H. Synthesis of high purity single-walled carbon nanotubes inhigh yield. Chemical Communications 2002:(22):2666-2667

Singh C, Shaffer M S P, Windle A H. Production of controlledarchitectures of aligned carbon nanotubes by an injection chemicalvapour deposition method. Carbon 2003:41(2):359-368

Laurent—Synthesis of carbon nanotubes-Fe-Al2O3 powders. Influence of thecharacteristics of the starting Al1.8Fe0.02O3 oxide solid solution, Ch.Laurent, A. Peigney, E. Flahaut, A. Rousset, MRS Bulletin, 35, 2000, pp661-673

Mauron—Fluidised-bed CVD synthesis of carbon nanotubes on Fe₂O₃/MgO,Diam ond and Related materials, Pages 780-785 Ph. Mauron, Ch.Emmenegger, P. Sudan, P. Wenger, S. Rentsch and A. Züttel

E. Flahaut, A. Govindaraj, A. Peigney, C. Laurent, A. Rousset, C. N. R.Rao, “Synthesis of Single-Walled Carbon Nanotubes using Binary (Fe, Co,Ni) Alloy anoparticles Prepared in Situe by the Reduction of Oxide SolidSolutions,” Chem. Phys. Lett., 300 (1-2) (1999) 236-242.

M. Su, B. Zheng and J. Liu, “A scalable CVD method for the synthesis ofsingle-walled carbon nanotubes with high catalyst productivity,” Chem.Phys. Lett. 322 (2000) 321-326.

1. A method of producing carbon nanoparticles, comprising the steps of:passing a gaseous carbon source through a heated reactor; and addingcatalyst supported on substrate particles or thermally decomposablecatalyst precursor supported on substrate particles to the heatedreactor; maintaining a fluidised bed of the substrate particles in theheated reactor; and forming carbon nanoparticles in the heated reactor.2. A method as claimed in claim 1, wherein the catalyst or catalystprecursor supported on substrate particles is introduced into the heatedreactor via a gravity-fed hopper.
 3. A method as claimed in claim 1,wherein the catalyst or catalyst precursor supported on substrateparticles is introduced into the heated reactor via an injection gasflow.
 4. A method as claimed in claim 3, wherein the injection gas flowreverses the direction of gas flow through the heated reactor duringinjection.
 5. A method as claimed in claim 3, wherein the injection gasis an inert gas.
 6. A method as claimed in claim 3, wherein theinjection gas is a gaseous carbon source.
 7. A method as claimed inclaim 1, wherein the reactor heated reactor is at a temperature between500 and 1200° C.
 8. A method as claimed in claim 7, wherein the heatedreactor is at a temperature between 700 and 900° C.
 9. A method asclaimed in claim 1, wherein a catalyst precursor is present and is ametal salt, an organometallic species or a metal carbonyl.
 10. A methodas claimed in claim 9, wherein the catalyst precursor comprises one ormore of nickel, iron, molybdenum, platinum and cobalt.
 11. A method asclaimed in claim 9, wherein the catalyst precursor is a metal salt andcomprises a counterion consisting of nitrate, stearate, formate,oxalate, acetate or chloride.
 12. A method as claimed in claim 11,wherein the counter ion is organic.
 13. A method as claimed in claim 12,wherein the organic counter ion is C₂ to C₃₀ carboxylate.
 14. A methodas claimed in claim 1, wherein the carbon nanoparticles contain anon-carbon dopant.
 15. A method as claimed in claim 14, wherein thenon-carbon dopant is nitrogen.
 16. A method as claimed in claim 1,wherein the gaseous carbon source is one or more of acetylene, alcohol,alkane, alkene, CO, benzene, toluene, xylene, cumene, ethylbenzene,naphthalene, phenanthrene, anthracene, formaldehyde, acetaldehyde,acetone.
 17. A method as claimed in claim 1, wherein the gaseous carbonsource is mixed with a diluent gas.
 18. A method as claimed in claim 17,wherein the diluent gas is one or more of hydrogen, ammonia, nitrogen,helium and argon.
 19. A method as claimed in claim 17, wherein the ratioof gaseous carbon source to diluent gas is reduced while the catalyst orcatalyst precursor supported on substrate particles is introduced intothe heated reactor.
 20. A method as claimed in claim 1, in which thesubstrate particles comprise one or more of silica, alumina, MCM andmagnesium oxide.
 21. A method as claimed in claim 1, in which thesubstrate particles comprise a halide, nitrate, sulphate, carbonate,aluminate, aluminium chloride, arsenate, arsenite, borate, chromate,fluoroaluminate, silicate, sulphide, telluride, tungstate, vanadate orphosphate of a Group 1 or Group 2 metal.
 22. A method as claimed inclaim 21, wherein the Group 1 or Group 2 metal is lithium, sodium,potassium, calcium or magnesium.
 23. A method as claimed in claim 1,wherein the average dimension of the substrate particles is between 20microns and 1 mm.
 24. A method as claimed in claim 1, wherein theaverage dimension of the substrate particles is between 40 microns and200 microns.
 25. A method as claimed in claim 1, further comprising thestep of removing nanoparticles from the heated reactor.
 26. A method asclaimed in claim 25, wherein nanoparticles are removed from the heatedreactor by under vacuum or under pressure.
 27. A method as claimed inclaim 1, wherein the process is operated continuously with simultaneousintroduction of catalyst or catalyst precursor supported on substrateparticles and removal of nanoparticles.
 28. A method as claimed in claim1, wherein the method is operated non-continuously with alternatingintroduction of catalyst or catalyst precursor supported on substrateparticles and removal of nanoparticles.
 29. A method as claimed in claim1, wherein the carbon nanoparticles are nanotubes and/or nanofibres. 30.A method as claimed in claim 29, wherein the nanotubes are single-wallednanotubes or multi-walled nanotubes.
 31. A method of producing carbonnanoparticles, comprising the steps of: passing a non-carbon-containinggas through a heated reactor; and adding catalyst or catalyst precursorsupported on substrate particles to the heated reactor; maintaining afluidised bed of said substrate particles in the heated reactor; passinga gaseous carbon source through the heated reactor; and forming carbonnanoparticles in the heated reactor.
 32. Carbon nanoparticles producedby a method as claimed in claim 1 or claim 31.